1. Technical Field
The present invention relates to a balun circuit and a frequency converting apparatus and, more particularly, the present invention relates to a balun circuit disposed between a first terminal, a second terminal, and a third terminal and a frequency converting apparatus that outputs a modulated signal obtained by shifting a frequency of a signal to be modulated according to a frequency of a local signal.
2. Related Art
A balun circuit is conventionally known as a circuit that generates a differential signal. For example, the balun circuit generates the differential signal through a signal output by two coupling lines connected in a cascading manner as in, for example, Japanese Patent Application Publication No. 2002-232215 (Patent Document 1).
Each coupling line includes an unbalanced line and a balanced line. The unbalanced line in each coupling line is disposed as a single continuous line. Furthermore, the balanced line in each coupling line is disposed parallel to the corresponding unbalanced line.
A signal transmitted by each unbalanced line is propagated to and received by the corresponding balanced line through magnetic coupling or the like and is then output to the outside. At this time, if a line length of each coupling line is a quarter of a wavelength of the transmission signals, differential signals differing in phase by 180 degrees are output from the coupling lines. Furthermore, the levels of the signals output by the balanced lines are determined by the levels of the signals transmitted by the corresponding unbalanced lines and an amount of coupling with the unbalanced lines.
A signal transmitted by the unbalanced line of a front coupling line is provided to a rear coupling line. Because of this, there is a case where the level of the signal provided to the front coupling line is different from the level of the signal provided to the rear coupling line because of signal decay or the like caused by the transmission. In such a case, when the coupling amount in each coupling line is set to be equal, the levels of the signals propagated to the balanced line of each coupling line are undesirably different. In other words, the signal levels of an inverted side and a noninverted side in the differential signal generated by the balun circuit are undesirably different.
In response to the problem of the levels of signals on the inverted side and the noninverted side being different, adjusting the levels of the signals output by each of the balanced lines by adjusting the coupling amount of each coupling line has been considered. For example, by adjusting the line widths of the unbalanced lines and the balanced lines included in the coupling lines, the coupling amount in the coupling lines is adjusted, so that the signal levels can also be adjusted.
However, there are cases where it is difficult to adjust the line width in each coupling line. For example, in a case where the balun circuit is formed on a semiconductor substrate or the like, it is difficult to dispose in a straight line a coupling line that has a line length that is one quarter of the wavelength of the transmission signal. In view of this, a prescribed line length can be ensured by forming each line of the coupling line in a spiral shape, a kinked line shape, or the like.
However, during formation of the wiring having a pattern that includes a curved portion or a kinked portion, it is desirable that the line width of the pattern wiring be uniform and smaller than a prescribed width. Therefore, with the method that adjusts the line width, the signal level of the inverted side and the noninverted side in the differential signal cannot be sufficiently adjusted.